The present invention relates to an image forming method and an image forming apparatus such as a copying machine, printer and facsimile, and in particular to an image forming apparatus configured in such a way that the photoconductor surface is exposed at the time of transfer of a toner image.
One of the well-known image forming apparatuses performs a method that comprises steps of: forming an electrostatic latent image on a uniformly charged photoconductor surface by exposure means; forming the electrostatic latent image into a toner image using development means with toner carried thereon; transferring the toner image onto a transfer material fed to a transfer area, from the photoconductor by transfer means; separating the transfer material from the photoconductor using separation means; feeding the transfer material to a fixing apparatus; fixing the toner image by application of heat and pressure; and ejecting the transfer material to a tray installed outside the apparatus by ejecting means.
It is also known that exposure is performed before or concurrently with transfer in order to improve transfer efficiency and separation performances when the toner image is transferred on the transfer material.
For example, a technique is disclosed, where light is applied from a light source provided inside a transfer electrode simultaneously with application of an electric field, and transfer is carried out while removing electric charges on a photoconductor (Patent Document 1).
However, the simultaneous transfer-exposure lamp disclosed in Patent Document 1 is arranged in such a way as to apply light to a range beyond the area where the electric discharge extends by the application of a voltage to the transfer electrode (hereinafter referred to as “transfer area”), namely, to the range outside the area close to the transfer electrode where an attraction force effectively acts on toner. Because of this configuration, due to reduction of potential, the toner absorbed on the surface of the photoconductor by electrostatic adsorption is put in an easily movable state, with the result that so-called scattering is likely to occur. (FIG. 6 shows an example where the simultaneous transfer-exposure lamp is applied to the point beyond the range where electric discharge from the transfer electrode generates effectively a suction force of toner. FIG. 6 will be explained later in details).
What is called scattering here refers to the condition where the transfer material and photoconductor are not brought into close contact with each other, and when the photoconductor is exposed through the transfer material and electric charge is eliminated during action of the transfer electrode, part of the toner is transferred from the photoconductor to a position where transfer should not be performed, with the result that disturbance (scattering) has occurred to the final image.
A method for avoiding the aforementioned scattering is found in the disclosed technology related to the image forming apparatus equipped with illumination range limiting means for ensuring that the exposed light of the simultaneous transfer-exposure apparatus where light is applied simultaneously with transfer will not be applied to the photoconductor before the transfer material is brought into contact with the photoconductor (Patent Document 2). The overview of Patent Document 2 is shown below:
(1) Provide illumination range limiting means for ensuring that the exposed light of the simultaneous transfer-exposure apparatus will not be applied to the photoconductor before the transfer material is brought into contact with the photoconductor.
(2) Provide alternating current charging means for ensuring that alternating current charging is applied to the photoconductor and toner prior to transfer.
FIG. 8 is a cross sectional view representing the example of the image forming apparatus disclosed in Patent Document 2. Numeral 1a denotes a transparent photoconductor belt. A simultaneous transfer-exposure lamp 6 and its lightproof plate 61 are provided inside the transparent photoconductor belt 1a. Arrangements are made in such a way the light of the simultaneous transfer-exposure lamp 6 is applied to the transparent photoconductor belt 1a simultaneously with transfer, after a transfer material P fed through transfer material feed path 11 comes into contact with the transparent photoconductor belt 1a. To put it another way, the light of the simultaneous transfer-exposure lamp 6 is blocked by the lightproof plate 61 before the transfer material P contacts the transparent photoconductor belt 1a so that light is not applied to the transparent photoconductor belt 1a. This structure prevents the aforementioned scattering phenomenon from occurring.
However, according to the aforementioned art (1), a lightproof plate 61 is provided to ensure that light of the simultaneous transfer-exposure lamp 6 will not be applied to the transparent photoconductor belt 1a before the transfer material P comes into contact with the transparent photoconductor belt 1a. After the transfer material P has come into contact with the transparent photoconductor belt 1a, the transparent photoconductor belt 1a is exposed before transfer discharge by a transfer apparatus 5 is applied to the transfer material P. Accordingly, electric charge is eliminated from the transparent photoconductor belt 1a during this time. This leads to toner scattering around the image.
According to the aforementioned art (2), electric charge on the transparent photoconductor belt 1a is removed by alternating current charging before transfer. Accordingly, scattering of toner to the surrounding area occurs, similarly to the case of the aforementioned art (1).
To reduce the scattering of toner, it is effective to reduce the amount of light of the simultaneous transfer-exposure lamp 6 and the amount of alternating current electrical charge before transfer. In this case, however, originally intended improvement of the transfer efficiency or separation performance cannot be achieved.
Especially when copying on the back side where transfer is performed on the transfer material corrugated by heat and pressure after passing through a fixing apparatus, or when it is comparatively difficult to get a close contact with the photoconductor as in the last end portion of the material and there is a large-scale corrugation of the transfer material, there are such problems as conspicuous scattering of toner at the time of transfer and bleeding of characters.
In a copying machine or printer of digital exposure type based on a laser and LED rapidly coming into widespread use in recent years, halftone images are often reproduced using a dot-based image. When toner has scattered around the dot forming a grid of dots, the image density appears high and this will lead to occurrence of uneven density in the halftone image.
When a toner image is formed on the back side after a toner image is formed on the front side of the transfer material, and a halftone image is formed by dots on the back side, toner scatters in the portion without toner on the front surface. Whereas the amount of light reaching the photoconductor is reduced in the portion with toner on the front surface and this results in reduced scattering of toner. And this portion appears pale, and a faulty image such as a so-called ghost will be produced.
[Patent Document 1]
Official Gazette of Jikkosho 1965-17412 (FIG. 3 on Page 1)
[Patent Document 2]
Official Gazette of Tokkaihei 1994-175440 (FIG. 1, paragraph 0016)
The object of the present invention is to solve the aforementioned problems and to provide an image forming method and an image forming apparatus, characterized by excellent transfer efficiency and separation performance, capable of preventing scattering of toner from occurring at the time of transfer and capable of getting a high-quality image, free from bleeding of characters or irregularity of density on the halftone image formed by dots, or ghost on the back side caused by the presence or absence of a toner image on the front side surface.